JPS638839B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention applies to non-decomposable materials such as flue gas desulfurization wastewater.
It concerns a biological treatment method for wastewater containing COD. Flue gas emitted from boilers and other combustion equipment using fossil fuels contains high concentrations of sulfur oxides and nitrogen oxides. If these are discharged into the atmosphere without treatment, they will cause many problems in terms of environmental conservation, both because of their own toxicity and because they change into even more harmful substances through photochemical reactions in the atmosphere. Therefore, in accordance with legal regulations, flue gas containing these substances is desulfurized and denitrated before being discharged into the atmosphere. One of the most commonly used flue gas desulfurization and denitration methods today is a method in which sulfur oxides and nitrogen oxides in flue gas are absorbed and removed by an alkaline solution. In this method, most of the sulfur oxides are removed as sulfates such as calcium sulfate and magnesium sulfate, but compounds that are difficult to be oxidized and decomposed such as dithionates are also produced as by-products. Among sulfur oxides that are difficult to decompose by oxidation, especially dithionic acid, decomposition by chlorine or ozone does not effectively decompose it, and physicochemical methods are mostly limited to methods using wet combustion. In view of the current situation, the present invention proposes a method for degrading the above-mentioned persistent COD components, which are sulfur oxides, by utilizing the biodegradation ability of microorganisms. The purpose of this invention is to provide a method that can accelerate the rate of biodegradation when decomposing substances. The present invention is a method for treating wastewater containing dithionic acid, which is characterized by adding organic matter to wastewater containing at least dithionic acid or a dithionate salt, and performing anaerobic biological treatment while removing generated hydrogen sulfide. The basic structure of the present invention will be explained in detail below based on the research history that led to the present invention. It is well known that sulfur-oxidizing bacteria (such as the genus Thiobacillus) and sulfate ring-forming bacteria (such as the genus Desulfoviburi) biodegrade dithionic acid and the like. This research also began with a simple continuous fermentation treatment method using anaerobic bacteria as an energy-saving treatment method for decomposing persistent COD components of sulfur oxides containing dithionic acid instead of the wet combustion method. Ta. The sample flue gas desulfurization wastewater is concentrated by dilution or physicochemical treatment such as ion exchange treatment.
Dithionic acid concentration is 100mg/asS ₂ O ^2- ₆ , 300mg/
asS ₂ O ^2- ₆ , 500mg/asS ₂ O ^2- ₆ , 1000mg/
Adjusted to 4 levels of asS ₂ O ^2- ₆ , and further added 1000 mg/(lactic acid) using lactic acid as a BOD source.
It was added and used so that it became. The inoculum used was a mixed bacteria cultured in an enrichment medium containing sodium dithionate from digested sludge obtained from an anaerobic digestion tank of sewage sludge, and the concentration in the experimental reaction tank was
Seeds were planted at 3000mg/・MLSS. As a result, the decomposition and removal of dithionic acid was almost completely achieved in the case of the sample wastewater with a dithionic acid concentration of 100 mg/day, but the dithionate concentration in the sample wastewater was
At 300 mg/dithionic acid, the removal rate of dithionic acid decreased to about 60%, and at dithionic acid concentrations of 500 mg/d or more, dithionic acid was hardly removed as days passed. As a result of investigating factors that inhibit the decomposition of dithionic acid, it was found that the hydrogen sulfide concentration in the treated water at the time when the inhibition phenomenon was observed changes depending on the dithionic acid concentration in the sample wastewater, as shown in Table 1. It was done.

[Table] The above experiment revealed that anaerobic biodegradation of dithionic acid cannot be continued by a simple anaerobic biodegradation method when the concentration of dithionic acid in the sample wastewater exceeds 100mg/ It is known that when this phenomenon occurs, the concentration of hydrogen sulfide in the reaction solution is relatively high. Next, dithionic acid concentration 300mg/,500
When we performed similar biological treatment experiments on wastewater with concentrations of 1000mg/mg/mg/mg/mg/mg/, and 1000mg/ of dithionic acid while desulfurizing the hydrogen sulfide concentration in the reaction solution to below 8mg/, we found that the test wastewater with all dithionic acid concentrations was almost completely cured. The decomposition treatment of dithionic acid was achieved. From the above results, it is known that in the anaerobic biodegradation treatment of dithionic acid, a high concentration of hydrogen sulfide is produced, which inhibits the decomposition of dithionic acid. The effectiveness of decomposition treatment while removing hydrogen has become clear. As described above, the method according to the present invention is particularly effective for wastewater with a high concentration of dithionic acid. It is preferable that at least dithionic acid is concentrated using a permeable membrane or the like. Next, an example of an embodiment of the present invention will be described in detail based on the drawings. Although the illustrated process formally belongs to the anaerobic contact method, other types of processes are also possible as described below. In the figure, the dithionic acid-containing wastewater discharged from the flue gas desulfurization process or the like or the concentration process is used as the influent 1 in an anaerobic biodegradation tank 4 (hereinafter referred to as
(abbreviated as decomposition tank). The residence time in the decomposition tank at this time is often set in the range of 0.5 to 10 days in the anaerobic contact method such as this embodiment, and the residence time of this wastewater is the amount of organic matter 15 added to the decomposition tank 4. It can be determined depending on the type. When using methanol as the additive organic substance, the residence time can be particularly shortened to a range of 0.3 to 2.0 days, while when using sewage sludge, industrial wastewater sludge, human waste, etc., the residence time can be reduced to 4 days. It is necessary to take a slightly longer period of 1-10 days. In addition to these organic wastewaters that have an additive effect on the decomposition of dithionic acid, there are municipal sewage, pit garbage juice at garbage treatment plants, sewage sludge or leachate discharged from municipal waste composting plants, and leaching from garbage landfills. Water, leachate from manure including domestic manure from pig pens and cow sheds and barn cleaning wastewater, molasses from sugar mills, steam drains from pulp mills, brewing and distillation wastewater from brewing plants, food processing For example, wastewater from factories can be used, and when these are used, the residence time in the reactor is 0.8 to 5.0 days, which is slightly longer than when methanol is used as the added organic substance. In addition, commercially available organic substances other than methanol that have an additive effect are alcohols such as propanol,
Butanol, glycerol, and ethanol can be mentioned, but among these, ethanol had a rather low addition effect. Among organic acids, additive effects were seen for formic acid, pyruvic acid, lactic acid, propionic acid, butyric acid, succinic acid, citric acid, and acetic acid. Particularly in the case of acetic acid, the addition effect was high when mixed with other organic acids. The reactor residence time for the addition of these organic acids is
Although it was necessary to make it about 0.6 to 4.0 days, which was slightly longer than in the case of methanol addition, it was found that it was shorter than in the case of adding the various waste sludge and wastewater. Also maltose,
Oligosaccharides such as cellobiose and glucose, and reducing sugars also required approximately the same residence time. The amount of organic matter to be added varies depending on the concentration of dithionic acid (salt) and other persistent COD substances in the wastewater to be treated, and the amount of organic matter to be added ranges from equivalent to the concentration of dithionic acid to 7 times the concentration of dithionic acid in terms of BOD. It is preferable to add it so that If the amount of added organic matter is less than this, dithionic acid will not be fully decomposed and undecomposed dithionic acid will remain in the treated water, and if more organic matter is added, the residual BOD concentration in the treated water will increase. due to aerobic treatment, etc.
BOD removal costs are high. In addition, in the present invention, the above-mentioned organic wastewater, organic sludge, alcohols,
It goes without saying that organic acids and saccharides can be used in appropriate combinations. Under the above-mentioned conditions of added nutrients and residence time, dithionic acid in the input wastewater is almost completely reduced and decomposed to hydrogen sulfide, and some remains as dissolved hydrogen sulfide in the tank liquid, but the rest remains. Transition to Digestion Gas Medium 14. The dithionic acid decomposition tank 4 of the present invention includes:
The temperature conditions used for normal methane fermentation, i.e. 20°C to 40°C for decomposition using mesophilic bacteria, and 45°C to 70°C for decomposition using thermophilic bacteria. Its efficiency can be significantly increased,
In particular, operation at 45° C. to 70° C. is more effective in reducing the dissolved H ₂ S concentration, but operation without temperature control may be used if decomposition efficiency is not an issue. The mixed liquid 2 flowing out from the decomposition tank 4 is sent to a vacuum pump 1
After being degassed by passing through the decompression chamber 5, which is depressurized by It is appropriate to use the same or weaker strength than the anaerobic contact method used for fermentation. In some cases, it is possible to omit the above degassing operation, but this selection must be made after confirming that the solid-liquid separability in the sedimentation separation tank 6 will not be extremely deteriorated. . In the sedimentation separation tank 6, the sludge is separated into a sedimentation (biological) sludge fraction 12 and a supernatant fraction 3, and this supernatant fraction 3 is treated as dithionic acid decomposition treated water.
It is either led to an aerobic treatment process to remove BOD components, etc., or directly discharged. A part of the settled sludge fraction 12 is returned to the decomposition tank 4 by the sludge transport pump 11, and the rest is disposed of as surplus sludge 13, but this quantitative ratio is such that the concentration of biological sludge in the decomposition tank 4 is 500 to 500. It can be adjusted and determined to be maintained at 10000mg/. Various methods can be used to remove hydrogen sulfide from the liquid in the decomposition tank 4. As shown in the figure, providing a gas circulation path for hydrogen sulfide accumulated in the digestion gas and providing an absorption device 7 in this path creates a low-concentration H ₂ S atmosphere in the gas phase of the head space of the decomposition tank, and allows the hydrogen sulfide to be absorbed into the liquid in the decomposition tank. This is done for the purpose of expelling residual H ₂ S. This H ₂ S absorption and removal agent may be a liquid absorbent such as a caustic soda solution, a ferrous solution, a calcium carbonate solution, a lime solution, an ammonia solution, or a monoethanolamine solution, or it may be a solid absorbent such as iron oxide. be. Furthermore, the Takahakus method and the Silock method, which have been conventionally used for desulfurization of digestion gas, can be used in some cases. The gas circulation route for hydrogen sulfide absorption is not necessarily limited to the method shown in the diagram; for example, the air supply pipe from the circulation pump (gas pump) 9 may be placed below the liquid level in the decomposition tank and used for gas agitation. This is preferable because the concentration of hydrogen sulfide dissolved in the liquid can be further reduced by the stripping effect than by gas circulation only in the head space. In addition to the above-mentioned gas absorption method, hydrogen sulfide can be removed by adding metal salts such as iron salts to the decomposition tank liquid to remove the hydrogen sulfide as insoluble sulfide. In any of these cases, it is preferable that the hydrogen sulfide concentration in the decomposition tank liquid is 20 mg/or less, and particularly preferably within the range of 0 to 8 mg/. If the pH in the decomposition tank shows extreme fluctuations, it is preferable to adjust the pH to a range of 5.5 to 7.5. This pH adjustment is effective not only in promoting the anaerobic biodecomposition reaction of dithionic acid, but also in removing the hydrogen sulfide through gas circulation. This is especially true when using a method that does not adjust the pH of the PH in the digester liquid.
becomes important when the value rises above 7.5. That is,
This is because at a pH higher than 7.5, the amount of dissolved hydrogen sulfide becomes significantly high, and hydrogen sulfide is not sufficiently gasified, making it impossible to keep the residual concentration of hydrogen sulfide in the liquid low. In addition, 8 in the figure is a stirrer. In addition to the above-mentioned embodiments, the semi-batch contact digestion method, which is an anaerobic digestion device conventionally known as a form of the anaerobic biological treatment device for flue gas desulfurization wastewater according to the present invention, and the upflow anaerobic filter bed method The fact that most of the equipment belonging to the anaerobic reactor method, fluidized bed anaerobic reactor method, upflow anaerobic sludge blanket method, etc. can be used is that the most efficient method can be used depending on the water quality of the flue gas desulfurization wastewater to be treated and the type of added organic matter. This can be cited as an advantage of widening the selection range when configuring expensive equipment. Note that after treating wastewater containing at least dithionic acid by the method of the present invention, if the water flowing out from the anaerobic reactor (decomposition tank 4) contains unused organic matter, hydrogen desulfurization is incomplete. If it is necessary to remove residual BOD or COD concentrations, such as when hydrogen sulfide is present or when sulfite ions are contained due to insufficient sulfate reduction, use an aeration tank or biofilm reactor (submerged filter bed) to remove residual BOD or COD concentrations. Needless to say, it is preferable to conduct the aerobic biological treatment by introducing the raw material into an aerobic biological treatment device such as a fluidized bed, rotating disk, trickling filter, etc.). As described above, the present invention is applicable to wastewater containing dithionic acid or dithionate, including sewage, human waste, various industrial wastewater, various agricultural and livestock wastewater, organic wastewater such as molasses, and organic wastewater discharged from these treatment processes. Waste sludge, commercially available organic substances such as methanol, ethanol, propanol, butanol, alcohols such as glycerol, oligosaccharides obtained by hydrolysis of polysaccharides such as cellulose and starch, sugars such as reducing sugars, and formic acid. , acetic acid, propionic acid, butyric acid,
A method of anaerobic biological treatment by adding one or a mixture of organic substances such as organic acids such as pyruvic acid, lactic acid, succinic acid, and citric acid to remove hydrogen sulfide generated in the biological treatment process. It is characterized by the fact that it can decompose recalcitrant COD substances in a short time, efficiently and energy-savingly through a simple process, and there is no problem of secondary pollution. It has many advantages, such as being able to use existing equipment as is or with only slight modifications.

[Brief explanation of the drawing]

The drawings are flow sheets illustrating embodiments of the invention. 1... Inflow liquid, 2... Outflow mixed liquid, 3... Supernatant liquid fraction, 4... Decomposition tank, 5... Decompression chamber, 6... Precipitation separation tank, 7... Absorption device, 8... Stirrer, 9...
...Circulation pump, 10...Reducing pressure pump, 11...Sludge transport pump, 12...Settled sludge fraction, 13...
Excess sludge, 14...digestion gas, 15...organic matter.

Claims (1)

[Scope of Claims] 1. A method for treating wastewater containing dithionic acid, which comprises adding organic matter to wastewater containing at least dithionic acid or a dithionate salt, and performing anaerobic biological treatment while removing generated hydrogen sulfide. 2. The method according to claim 1, wherein the waste water is concentrated water of dithionic acid or dithionate salt, and is obtained by concentrating flue gas desulfurization waste water by physicochemical treatment such as ion exchange treatment. 3 As the organic matter, organic wastewater, organic sludge,
3. The method according to claim 1 or 2, wherein a substance arbitrarily selected from the group consisting of alcohols, organic acids, and saccharides is used. 4. The method according to claim 3, wherein methanol is used as the organic substance. 5. The method according to claim 3, wherein acetic acid is used as the organic substance. 6 The amount of added organic matter is calculated based on the BOD equivalent amount of dithionic acid, dithionate, and other non-decomposable substances.
Claim 4 or 5 sets the amount to be 1 to 7 times the COD equivalent amount of the COD substance.
The method described in section. 7. The method according to claim 6, wherein the anaerobic biological treatment is carried out while maintaining the hydrogen sulfide concentration of the biological reaction liquid at 20 mg/or less, preferably 8 mg/or less. 8 Perform the anaerobic biological treatment and adjust the pH of the biological reaction solution.
The method according to claim 6 or 7, which is carried out by adjusting the ratio to 5.5 to 7.5. 9 Perform the anaerobic biological treatment by adjusting the temperature of the biological reaction solution.
Claim 6,
The method described in paragraph 7 or 8.